The present invention relates to diagnostic systems for vehicles, and more particularly to a method and apparatus for monitoring catalytic converter efficiency and secondary air injection.
During the combustion process, gasoline is oxidized, and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a three-way catalytic converter that reduces CO, HC and NOx in the exhaust gas. The efficiency of the catalytic converter is periodically monitored to prevent excess CO, HC and NOx in the exhaust gas. Typically, the catalytic converter is monitored during engine steady state conditions. At idle, for example, the engine controller adjusts the air to fuel (A/F) ratio to achieve consistent emissions output. Traditional monitoring methods force the A/F ratio to a lean or rich condition for a predetermined period. Afterwards, the controller switches to the rich or lean condition. The controller estimates an oxygen storage capacity (OSC) of the catalytic converter based on a lag time between an inlet oxygen sensor and an outlet oxygen sensor detecting the lean/rich condition. The OSC is indicative of the efficiency of the catalytic converter.
The intrusive catalytic converter monitoring test adversely impacts emissions and driveability. For example, operation in a lean A/F ratio may cause engine instability. Compensation involving more intrusive control of other engine parameters is typically required to prevent engine instability.
A secondary air injector may also be provided to inject air into the exhaust stream. The secondary air injector normally operates during a short start-up period of the engine. During the startup period, the engine is still xe2x80x9ccoldxe2x80x9d and combustion of the gasoline is incomplete, which generates dense emissions, especially CO and HC. Additional air injected by the secondary air injector is used to quickly heat the catalyst by oxidizing the CO and HC. The warmed catalytic converter further oxidizes CO and HC, and reduces NOx, to lower emissions levels.
Accordingly, the present invention provides method and apparatus for monitoring both a secondary air injector and a catalytic converter. The method includes operating an engine in a rich condition after detecting an engine steady state condition. The secondary air injector injects air into an exhaust stream to create a lean condition. The injection of the air into the exhaust stream is ceased after both inlet and outlet sensors detect the lean condition. After ceasing air injection, a lag time is measured between the inlet sensor detecting the rich condition and the outlet sensor detecting the rich condition. An oxygen storage capacity of the catalytic converter is calculated based on the lag time. An efficiency of the catalytic converter is determined as a function of the storage capacity. Additionally, a secondary air injector fault is signaled if the inlet sensor fails to detect the lean condition.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.